A known type of container-closure combination is a cap having a non-gas tight screw thread fitting with a complementary threaded neck of a container and a sealing element in the cap to form a gas- and liquid-tight seal with the container neck. Such a combination is illustrated by Document CH-A-357330. Liquid containers can become over or under pressurised and the container damaged, such as by ballooning or crushing, depending on the liquid to be contained and the ambient temperatures. One solution is to make the container strong enough to resist such changes. Another solution is to fit the container with a gas vent. The choice of solution is mainly an economic one, depending upon whether or not it is cheaper to make the container stronger or to fit a gas vent, although sometimes environmental considerations are a factor.
Document WO-A-95/26913 discloses a cap lining for bi-directional venting from the interior of a container to the ambient atmosphere through openings existing between the spiral screw threads of the cap closure and threads of the container neck. The only seal disclosed between the cap and the container is that provided by compression of the cap lining plies. Such special multilayer cap linings are however quite expensive to manufacture. An alternative is to provide a hole in a central region of an otherwise standard cap sealing wad over which a gas permeable, liquid impermeable membrane is secured. A further hole is formed in the centre of the cap top wall, in gas communication with the wad hole and membrane, thereby providing a gas venting route. However the exposed hole in the top of the cap is vulnerable to dirt and other contaminants. These can clog the membrane and inhibit proper venting or penetrate the membrane and contaminate the container contents, unless further protective structures are provided in the wad and/or cap top surface. The exposed hole in the top of the cap also makes the membrane vulnerable to mechanical damage. There is therefore a need for a cheap, robust, versatile and reliable container venting arrangement which preferably requires minimal modifications to the container closure and/or sealing wad.